A survey of the YAN status of South African grape juices and exploration of multivariate data analysis techniques for spectrometric calibration and cultivar discrimination purposes.

Petrovic, Gabriella (2018-12)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Yeast assimilable nitrogen (YAN) has been identified as one of the main drivers of wine quality, influencing the production of various aromas and ensuring a successful fermentation to dryness. Due to the number of factors affecting YAN concentration and composition, paired with the complexities of yeast metabolism, more data is required to enable a comprehensive understanding of this important component of the grape juice matrix. Thus, there is a need for simple, rapid, and cost-effective methods to measure YAN status. The main aims of this research were to gain insight into the nitrogen status of grape juices used for commercial winemaking in the South African wine industry, and subsequently, to assist in a more comprehensive understanding of grape juice nitrogen status. Therefore, in Chapter 3, an unsupervised survey of the YAN, FAN, and ammonia concentrations of 805 grape juice samples of various (industrially relevant) cultivars and geographical origins are reported. Subsequently, an overall average of 191 ± 64 mg N/L, 138 ± 46 mg N/L and 53 ± 24 mg N/L was observed for YAN, FAN, and ammonia, respectively. Trends of nitrogen deficiency and excess could be found for various cultivars and geographical origins. Analysis of variance tests and exploratory data analysis techniques such as hierarchical agglomerative clustering and CART analysis established ‘cultivar’ as the most important factor in determining the YAN concentration and composition of the resulting grape juice. In Chapter 4, using the data collected in Chapter 3, plus an additional vintage (2018), the viability of infrared (IR) spectroscopy for the accurate quantification of YAN, FAN, and ammonia was tested. IR spectroscopies compared included: Fourier-transform infrared (FT-IR), Fourier-transform near infrared (FT-NIR) and attenuated total reflection mid-infrared (ATR-MIR) spectroscopy. FT-IR and FT-NIR were found to outperform ATR-MIR in a variety of tasks assigned to each instrument and were deemed robust and capable of accurate quantification as RPDVAL > 2.5 were repeatedly obtained for both spectroscopies. The achievement of accurate calibration models is owed to the large amount of variability included in both the calibration and validation sets and the application of proper external validation strategies. Thus, both industry and research are presented with a simple, rapid and cost-effective method to measure this important component of the grape juice matrix. In Chapter 5, a deeper look into the FAN component of YAN was conducted by quantifying individual amino acids. Overall, proline, arginine, glutamine, alanine, tryptophan and GABA were found to be the most abundant while glycine, lysine, methionine and, ornithine were found to be the least abundant. Subsequently, the discriminatory power of the amino acid profile of the various cultivars were tested. This was done to identify key differences in amino acid profiles which could possibly serve as the basis for further research investigating yeast metabolism and aroma production during fermentation. The results of this research have contributed a wealth of information regarding the nitrogen status of various cultivars of Vitis vinifera, together with a rapid and easy-to-use method for the quantification of the nitrogen status of the grape juice matrix. This was done in hope of furthering the research efforts in this field to aid the production of quality wines, capable of meeting consumer demands.

AFRIKAANSE OPSOMMING: Gis-aanneembare stikstof (YAN) is geïdentifiseer as een van die belangrikste dryfkragte van wynkwaliteit, wat die produksie van verskillende aromas beïnvloed en 'n suksesvolle fermentasie-tot-droogte verseker. As gevolg van die aantal faktore wat die konsentrasie en die samestelling van YAN beïnvloed, tesame met die kompleksiteite van gismetabolisme, word meer data benodig om 'n omvattende begrip van hierdie belangrike komponent van die druiwesapmatrikste te bewerkstellig. Daar is dus 'n behoefte aan eenvoudige, vinnige en koste-effektiewe metodes om YAN-status te meet. Die hoofdoelwitte van hierdie navorsing was om insig te verkry van die stikstofstatus van druiwesap, wat gebruik word vir kommersiële wynmaak in die Suid-Afrikaanse wynbedryf. Die studie beoog verder om 'n meer omvattende begrip van druiwesap-stikstofstatus ten toon te stel. Hoofstuk 3 lewer resultate oor die opname van die YAN-, FAN- en ammoniak-konsentrasies van 805 druiwesapmonsters van verskeie kultivars en geografiese distrikte. 'n Algehele gemiddeld van 191 ± 64 mg N/L, 138 ± 46 mg N/L en 53 ± 24 mg N/L is waargeneem vir onderskeidelik YAN, FAN en ammoniak. Neigings van stikstof-tekorte en -oormaat is gevind vir verskillende kultivars en verskillende geografiese distrikte. Analise van variansie toetse en ‘n verskeidenheid data analise tegnieke soos hiërargiese agglomeratiewe ‘clustering’ en CART analise het 'kultivar' as die belangrikste faktor in die bepaling van die YAN konsentrasie en samestelling van druiwesap bevestig. In hoofstuk 4 word die data van hoofstuk 3 gekombineer met ‘n addisionele oesjaar (2018), om die geskiktheid van infrarooi (IR) spektroskopie vir die akkurate kwantifisering van YAN, FAN en ammoniak te toets. Die IR-spektroskopieë wat vergelyk is sluit in: Fourier-transform infrarooi (FT-IR), Fourier-transform nabye infrarooi (FT-NIR) en verswakte totale refleksie mid-infrarooi (ATR-MIR) spektroskopie. Daar is gevind dat FT-IR en FT-NIR herhaaldelik beter presteer as ATR-MIR in 'n verskeidenheid take – vir beide FT-IR en FT-NIR is RPDVAL> 2.5 konstant verkry. Die akkuraatheid van die kalibrasie modelle kan toegeskryf word aan die groot hoeveelheid veranderlikes wat ingesluit is in beide die kalibrasie- en valideringsstelle, tesame met die toepassing van behoorlike eksterne valideringstrategieë. Dus, die modelle bied 'n eenvoudige, vinnige en koste-effektiewe metode aan die industrie en die akademie om hierdie belangrike komponent van die druiwesapmatriks te meet. In Hoofstuk 5 is 'n meer in-diepte ondersoek na die FAN-komponent van YAN gedoen deur individuele aminosure te kwantifiseer. Daar is grotendeels bevind dat proline, arginien, glutamien, alanien, tryptofaan en GABA in die hoogste konsentrasies voorkom; terwyl glikien, lysien, metionien en ornitien in die laagste konsentrasies voorkom. Vervolgens is die onderskeid in die aminosuurprofiel gebruik om die verskillende kultivars te identifiseer. Dit is gedoen om sleutel verskille in aminosuurprofiele te identifiseer wat moontlik kan dien as die basis vir verdere navorsing van gismetabolisme en aroma-produksie tydens fermentasie. Die resultate van hierdie navorsing het tot 'n wye basis van inligting bygedra rakende die stikstofstatus van verskeie kultivars van Vitis vinifera, sowel as 'n vinnige en maklike metode om die stikstofstatus van die druiwesapmatriks te kwantifiseer. Dié studie is gedoen om die navorsingspogings in hierdie veld te bevorder en die produksie van gehalte wyne te bereik wat sal voldoen aan verbruikersbehoeftes.

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